Single photon sources, which can deliver one photon at a time upon demand of a user, have a wide range of applications especially in quantum information technologies. For example, in quantum key distribution, single photons delivered by single photons sources can act as quantum bits (qubits), which store information in their polarization state or phase. Suppressing multi-photon states can be important in quantum key distribution, because a multi-photon state is susceptible to a photon number-splitting attack. Single photon sources can also be used in some implementations of quantum computers, which allow the solution of problems that cannot be solved efficiently by classical computation. In quantum lithography, the coherence of an n-photon number state, which can be produced by combining n single-photon states, can achieve an n-fold increase in the resolution of an interferometric measurement, compared to the Rayleigh resolution limit obtained using a classical beam containing n photons. In addition, single photons could also be useful in performing sensitive absorption measurements (e.g., quantum radiometry).
Existing single photon sources attempt to deliver single photons via various mechanisms, but each has its own drawbacks. For example, attenuated coherent light (e.g., a laser), which obeys Poisson statistics, may emit single photons by tuning the mean photon number to be one from a statistical point view. However, the fluctuations about that mean photon number can impair the repeatability of the source. In another example, single quantum dots in III-V and II-VI semiconductor heterostructures, or single trapped atoms and ions, can also be used as single-photon emitters, but most of these sources operate at cryogenic temperatures. Color centers combined with vacancy centers in diamond, such as N-vacancy centers, Ni-N complexes, Si-vacancy centers, and Xe-vacancy centers may also be employed to emit single photons, but the collection efficiency of these diamond-based sources is generally very low.